Functionalized hydrogenated elastomers offer the advantage of higher mechanical strength and higher levels of adhesion to a variety of substrates. Hydrogenated carboxylated nitrile rubber is an example of a functionalized elastomer that has been extensively studied using conventional noble metal-catalyzed hydrogenation processes, but very few studies have been conducted using the diimide reduction process.
The diimide reduction process comprises reacting an ethylenically unsaturated polymer in latex form with a reducing agent, an oxidant, and a metal catalyst. The diimide reduction process for unsaturated elastomers has been studied by chemists for more than 20 years. It was originally developed by Lawson G. Wideman at The Goodyear Tire & Rubber Company and was first described in U.S. Pat. No. 4,452,950. The diimide reduction process offers a number of advantages compared to conventional metal-catalyzed hydrogenation processes. For example, the diimide reduction can selectively hydrogenate double bonds without reducing functional groups, such as carboxyl groups, hydroxyl groups, epoxy groups, amide groups, and active halide functional groups, which makes it possible to produce different functionalized hydrogenated polymers. This process also has an advantage over metal-catalyzed hydrogenation, in that high-pressure reactors are not needed, and the desired hydrogenation reactions can be conducted at atmosphere pressure.
However, the diimide reduction process described in U.S. Pat. No. 4,452,950 has some deficiencies that excludes the used of elastomers hydrogenated using it in some applications. The most serious of these deficiencies is the result of a cross-linking reaction that occurs during the hydrogenation process. This cross-linking severely limits the number and types of applications in which hydrogenated elastomers made using the diimide reduction technique can be employed. This problem is compounded by the fact that further cross-linking occurs during subsequent coagulation steps which often renders the hydrogenated elastomer unsuitable for use in many applications. In view of these deficiencies, a great deal of research has been conducted over the years in an attempt to develop a technique to limit gel formation in elastomers that are hydrogenated using the diimide reduction process.
U.S. Pat. No. 5,039,737 reveals a process for treating hydrogenated lattices made utilizing the diimide reduction technique with ozone to reduce the level of gelled (crosslinked) polymer and to reduce the level of residual hydrazine present. U.S. Pat. No. 5,039,737 more specifically discloses a process for treating an emulsion of a crosslinked elastomeric polymer containing residual hydrazine to obtain a latex of soluble (uncrosslinked) elastomeric polymer having a reduced concentration of residual hydrazine therein, said process comprising the addition of ozone to said emulsion in an amount and under conditions which are sufficient for the ozone to react with the crosslinked elastomeric polymer and the residual hydrazine to produce a latex of soluble (essentially uncrosslinked) elastomeric polymer having a reduced level of residual hydrazine therein. However, this ozone treatment process is extremely difficult to implement and control in commercial applications. Additionally, the ozonolysis breaks double bonds in the polymer and results in treated polymer having terminal aldehyde end groups. In any case, it changes the polymer structure and some microgel still typically persists. Thus, this ozone treatment technique is not universally suitable in commercial applications.
U.S. Pat. Nos. 5,424,356, 5,442,007, and 5,442,009 disclose a technique for addressing the shortfalls associated with the ozone treatment process. This technique involves treating the ozonated latex with hydroxylamine in an amount and under conditions which are sufficient to convert the aldehyde end groups of the elastomeric polymer in the latex to oxime end groups. This results in the formation of an oximated polymer latex. Unfortunately, this process changes the structure of the polymer and microgel still continues to persist after treatment. Accordingly, this process is again not commercially viable in most applications.
U.S. Pat. No. 6,521,694 describes using a boron type catalyst in the diimide reduction process. The process described by U.S. Pat. No. 6,521,694 utilizes (1) a reducing agent selected from hydrazines and hydrazine-releasing compounds, (2) an oxidising compound, and (3) a catalyst, wherein the catalyst contains an element from group 13 of the Periodic Table of the Elements, such as boron. It is preferred for the catalyst to be chosen from the group consisting of borates, peroxiborates and boric acid (H3BO3). More preferably, the catalyst is boric acid. It is most preferred that boric acid is used in combination with a polyvinyl alcohol. Crosslinking and resulting gel formation are problems that also occur in cases where this type of catalyst system is implemented. Thus, the technique of U.S. Pat. No. 6,521,694 has limited commercial applicability.
U.S. Pat. No. 6,552,132 discloses a process for the hydrogenation of a polymer composed of diene monomer units and nitrile group containing monomer units, in which the hydrogenation in the form of an aqueous dispersion is carried out in the presence of hydrazine and an oxidizing compound, comprising breaking crosslinks formed as a result of the hydrogenation by adding after the hydrogenation a compound that satisfies formula I or by adding before, during or after the hydrogenation a compound that satisfies formula
where
R1 is a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms and
R2 is an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms,    X is selected from the group consisting of —R3, —OR4, —SR4, and —NR5R6, where
R3, R4, and R5 are a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms and
R6 is an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms,    Y is selected from the group consisting of —R7, —OR8, —SR8, —NR9R10, and —N═CR11R12, where
R7, R8, R9, R10, R11, and R12 are a hydrogen atom, an alkyl or cycloalkyl group with 1-30 carbon atoms, or an aromatic group with 6-30 carbon atoms,
and wherein R3-R12 optionally contain one or more heteroatoms from the groups 13, 14, 15, 16, or 17 from the Periodic System of the Elements, and whereby the process is carrying out without using ozonolysis to break the crosslinks. However, the compounds used in this patent have limited effectiveness.
There is a continuing need for a technique to make hydrogenated lattices of elastomeric polymers without causing gel formation or changing the attributes of the polymer, such as by introducing unwanted functionality. There is a particular need for hydrogenated lattices of low molecular weight functionalized elastomers. For instance, functionalized hydrogenated rubber latex is of value in making resorcinol formaldehyde latex and fiber composites. It also can be coagulated and dried for utilization as a reactive functionalized liquid polymer for coatings, adhesives and polymer additives.